feat: comprehensive debug logging across all controller modules

Add detailed [DEBUG] logging to every controller module when
logging.level is set to "debug". Each module with stateful debug
uses SetDebug(bool) wired from main.go. Covers stacks, backup,
cloudflare, integrations, system, monitor, settings, scheduler,
web handlers, storage, metrics, API, selfupdate, and assets.

Also includes the app export/import (.fab bundles) feature from
v0.32.0 and its debug page integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/system/info.go

@@ -1,5 +1,19 @@
 package system
 
+import "log"
+
+// DebugLogger, when non-nil, enables debug-level logging for the system package.
+// Set this to the application's *log.Logger when config logging.level == "debug".
+// When nil, no debug output is emitted.
+var DebugLogger *log.Logger
+
+// debugf logs a formatted message if DebugLogger is set.
+func debugf(format string, args ...any) {
+	if DebugLogger != nil {
+		DebugLogger.Printf(format, args...)
+	}
+}
+
 // SystemInfo holds system resource usage information.
 type SystemInfo struct {
 	TotalMemMB  uint64  `json:"total_mem_mb"`